1. Field of the Invention
This invention relates to a digital signal transmission system for effecting time-division multiplexing/circuit switching data transmission by the use of a transmission cable.
More particularly, this invention relates to a digital signal transmission system effecting by an improved method the synchronization of system timing required for the purpose of controlling the timing of issuance of packet signals from individual personal stations for transfer on a transmission cable.
2. Description of the Prior Art
The dissemination of electronic computers and the growth of digital signal processing techniques have recently culminated in successful combination of communication systems with data processing systems and perfection of data communications devoted to on-line data processing. These achievements are attracting mounting public interest and respect.
Particularly in the small-scale communication systems such as for the intraorganizational communication confined within the premises of a government or public office or of a private corporation, the system which effects communication in the form of packets by means of a transmission cable such as a coaxial cable is ar esting particularly keen interest owing to its features such as good economy, high reliability, and prominent efficiency of transmission.
This communication system which makes use of packets requires installation, as in laboratories, of transmission cables adapted to effect transmission in both directions and connection to these transmission cables of numerous personal stations. From these personal stations, messages divided into data blocks each of 1000 to 2000 bits, for example, are transmitted through the transmission cables. The individual messages are each prefixed with a header covering such information as address and serial number.
In the communication system of this nature, the network itself is a passive transmission medium totally devoid of any control function and the individual personal stations have such control functions thoroughly distributed among themselves.
At a given personal station, therefore, transmission of a message is started when an idle channel is available in the cables. When a packet of message transmitted from one personal station collides with a packet of message transmitted from another personal station, these two personal stations discontinue the transmission of their messages. The personal station which has discontinued the transmission, on elapse of a random queuing time, tries to resume the transmission of the message.
In the communication system of this operating principle, users posted near the personal stations enjoy access to a common computer to which the communication system is interfaced. They are also able to have common use, through the medium of this communication system, of various items of hardware such as memory devices and various items of software such as programs distributed throughout the organization.
The devices such as high-speed or high-resolution printers and large-capacity files which are concentrated at the large-scale central processing unit in the time-sharing system can be used in the communication system as widely distributed within the premises of a given organization.
The communication system, therefore, enables the user to enjoy economization of resources and improvement in efficiency of utility. Besides, it warrants ample flexibility of programs and data and promises development of an intensive software system.
Further, the communication system of this nature has an advantage that all the personal stations are equally entitled to the use of channels, namely no special personal station has priority on the use of a channel over the remainders. This means that the present communication system does not have the hierarchical relationship often found among the personal stations in communication systems of other operating principles, so that communication can be established between two freely connected personal stations.
It has another advantage that the system can be easily designed in a highly reliable network because the component channels such as of coaxial cables are invariably formed of completely passive circuits.
Although this communication system enjoys various features as described above, it has a possibility of suffering packets issuing from different personal stations to collide with each other on one and the same channel because the individual personal stations of the system are allowed to start sending out data at any time desired. The frequency of the collision between packets will naturally become conspicuous in proportion as the system's efficiency in the use of channels is heightened.
As a solution of this problem, the signal transmission systems called Priority Ethernet and Reservation Ethernet have been proposed [by Japanese Patent Application No. SHO 56(1981)-38714 and No. SHO 56(1981)-172895, for example].
The Priority Ethernet system, by means of preambles in packets, assigns degrees of priority to signals to be transmitted from individual personal stations. If two packets from different personal stations happen to collide with each other, the signals in a packet of higher priority are allowed to be transmitted preferentially over those in the other packet of lower priority.
The Reservation Ethernet system maintains a master station for designation of modes and causes this master station, while in the reservation mode, to check all the other personal stations and confirm whether or not they have signals to be transmitted and, when certain personal stations are found to have signals to be transmitted, examine such personal stations and determine the amounts of their data for transmission.
On the basis of the results of this determination, the master station fixes the sequence in which the relevant packets will be transmitted by individual personal stations within a given frame and, in the transmission mode, causes the signals to be transmitted in the time-division pattern.
In accordance with the Priority Ethernet system, however, there still persists the problem that among a plurality of packets of equal degree of priority, dispersion is caused in the transmission delay time by collision.
Owing to this problem, this system proves unsuitable for real-time signal transmission such as for example, the voice transmission in the form of a conversation which attaches the greatest emphasis to the real-time relationship of response between transmission and reception.
In accordance with the Reservation Ethernet system, the presence of the master station deprives the personal stations of their mutual equality. This system suffers from poor reliability of performance because it has the possibility of ceasing to provide required data communication when the master station runs into trouble.
For the solution of the problem just mentioned, there has been proposed the modified Ethernet system which is capable of providing real-time transmission without depriving the personal stations of their mutual equality.
In accordance with this signal transmission system, the frames which are periodically repeated along time axis are each
In accordance with this signal transmission system, the frames which are periodically repeated along time axis are each divided into a plurality of blocks along the time axis. With these blocks as the unit, this system provides the personal stations with chances for packet type communication.
In this signal transmission system, all the personal stations are equally entitled to the use of idle blocks. In case where a given personal station occupies a specific block over a duration necessary for signal transmission, that personal station is periodically given a chance for signal transmission in each of frames repeated on the time base. Thus, this system permits the personal stations to effect real-time transmission of signals by making use of the function described above.
One typical frame configuration for the signals to be used in the aforementioned digital signal transmission system is illustrated in FIG. 1.
Each of the frames which are repeated periodically on the time base consists of N blocks (#1 through #N). And each of the blocks consists of various bit rows, b.sub.1 through b.sub.9, as shown below.
b.sub.1 . . . Rear guard time PA0 b.sub.2 . . . Preamble PA0 b.sub.3 . . . Address bit PA0 b.sub.4 . . . Distance code bit PA0 b.sub.5 . . . Control bit PA0 b.sub.6 . . . Data bit PA0 b.sub.7 . . . Check bit PA0 b.sub.8 . . . End flag PA0 b.sub.9 . . . Front guard time
The bit rows b.sub.2 through b.sub.5 and the bit rows b.sub.7 and b.sub.8 are essential components for a packet. These bit rows are collectively referred to as "overhead bits." The two bit rows, b.sub.1 and b.sub.9, are collectively referred to as the "guard time."
The term "guard time" means "empty bit rows" which are intended to preclude the situation in which packets in adjacent blocks may possibly be caused to overlap, if partially, owing to the delay time which occurs during the propagation of signals on a coaxial cable.
In the bit rows forming this guard time, the rear guide time b.sub.1 serves to protect the trailing one of any two adjacent packets against the trouble of overlapping and the front guard time b.sub.9 similarly to protect the leading packet against the trouble.
The sum of the number of bits of the rear guard time b.sub.1, and that of bits of the front guard time b.sub.9, will be represented as g bits and the guard time (b.sub.1 +b.sub.9) will be represented hereinafter as T.sub.g.
In the digital signal transmission system proposed as described above, when none of the personal stations in the system is transmitting signal, all the personal stations have a chance, equally and at any time at all, to start sending out signals in the aforementioned frame configuration. Thus, the particular personal station which is the first to start sending out signal onto the transmission cable will take the initiative in the synchronization of frames.
Once the frame synchronization has been established as described above, all the personal stations are enabled to keep watch on the state of signals being transmitted on the transmission cable.
As will be described fully afterward, the user devices at the personal stations are each provided with a memory adapted to memorize the condition of occupation of individual blocks by signals in the frames. Thus, all the personal stations are allowed to register relevant blocks based on the incoming packet signals addressed to themselves.
After the particular personal station has established the frame synchronization, any of the other personal stations is allowed to send out packet signals by selecting empty blocks based on the information stored in the aforementioned memory and loading these empty blocks with packet signals desired to be transmitted.
In this case, the timing by which the personal stations are allowed to send out their own packet signals poses a problem.
For the sake of explanation, let us assume that, as illustrated in FIG. 2, a coaxial cable 3 has its opposite ends connected to impedance matching terminators 1, 2, a personal station C is located at the middle point of the coaxial cable 3, and a personal station S located between the personal station C and the terminal 1 is already in the process of transmitting signals on the coaxial cable 3.
In this case, the packet signals which are being sent out by the personal station S are received by the personal station C and the other personal stations, R.sub.1 through R.sub.4, on the coaxial cable 3 at different points of time, depending on the variation in the signal propagation delay time on the cable 3.
If the personal stations randomly send out their own signals without paying any respect to the other personal stations, then there is a fair possibility that the packets issuing from such personal stations will overlap (collide with) each other on the coaxial cable 3.
For the purpose of precluding this detestable phenomenon, the aforementioned signal transmission system makes effective use of the aforementioned concept of guard time T.sub.g, in establishing the synchronization of system timing.
To be more specific, in this signal transmission system, the guard time T.sub.g, is fixed at two or more times of the signal propagation delay time required to cover the distance between the centrally located personal station C, datum position, and the most distant personal station and the transmission of signals is effected so that, at the receiving point of the centrally located personal station C, the packets issuing from the individual personal stations will be arranged as separated by equal intervals.
FIG. 3 provides more specific illustration of the working of the signal transmission system. The diagram depicts the system on the assumption that while the personal stations of the system are connected as illustrated in FIG. 2, the personal station S is already in the process of transmitting signals and the other personal stations, R.sub.1 through R.sub.4, are about to start sending out packet signals.
In this case, the personal stations R.sub.1 through R.sub.4 which follow the personal station S in the order of signal transmission determine their own points of timing for sending out their own transmission packets so that the personal station C, the datum point, will begin to receive the transmitted packets one guard time T.sub.g, after the personal station C completes reception of the transmission packets (or transmission S packets) from the personal station S.
To determine such timing for the issuance of signals, the personal stations R.sub.1 through R.sub.4, on receiving the packet signals transmitted on the coaxial cable 3, first examine the address bits (b.sub.3) of the received packet signals and discern the reception of packets from the personal station S (reception S packets).
Further, the personal stations R.sub.1 through R.sub.4, based on the signal propagation delay times between the personal station S, the centrally located personal station C, and their own stations, determine the points of time at which the arrival of reception S packets at the point of reception of the personal station C is completed.
These points of time, as illustrated in FIG. 3, are later than the points of time at which the reception of the reception S packets at the personal stations R.sub.1 and R.sub.2 is completed and earlier than the point of time at which the reception of the reception S packets at the personal stations R.sub.3 and R.sub.4 is completed.
After the personal stations R.sub.1 through R.sub.4 have determined the point of time at which the reception of reception S packets is completed with reference to the personal station C as the datum point, the particular one of these four personal stations which desired to send out signals begins to send out packet signals [or transmission R.sub.i packets (i=1 to 4)] at the point of time which is earlier than the determined point of time mentioned above by an interval equivalent to the signal propagation delay time required to cover the distance between its own station and the personal station C.
The packet signals which have been sent out as described above begin to be received (as reception R.sub.i packets) at the personal station C as the datum point after elapse of one guard time T.sub.g, from the time of completion of the reception of reception S packets as illustrated in FIG. 3.
This adjustment of the timing for sending out signals is accomplished by establishing frame synchronization and block synchronization at all the personal stations concerned.
Specifically, the personal stations are adapted, as described fully afterward, to reset periodically at a fixed timing the frame counter and the block counter which take count of the clock signals fed out by their own oscillators. Because of this function, the personal stations are able to establish frame synchronization and block synchronization within the tolerance of frequency of the clock signals.
In a system constructed as illustrated in FIG. 2 and FIG. 3, for example, while the personal station S is transmitting S packets by using the first block #1, the personal stations R.sub.1 through R.sub.4 control the aforementioned counters and establish the frame and block synchronizations so that the second block #2, for example, will be started after elapse of the time T.sub.b9, of the formula (1): EQU T.sub.b9 =T.sub.g/2 +.vertline.T.sub.g/2 -T.sub.SO .vertline.-.vertline.T.sub.SO -T.sub.RiO .vertline. (1)
Wherein i=1 to 4
from the point of time at which the reception of reception S packets is completed. The time T.sub.b9 is hereinafter referred to as "front guard time".
In the aforementioned formula (1), T.sub.SO denotes the signal propagation delay time required to cover the distance between the reference end of the coaxial cable 3 (such as, for example, the terminal 2 in the diagram of FIG. 2) and the personal station S and T.sub.RiO denotes the signal propagation time required to cover the distance between the aforementioned end to the personal station R.sub.i (i=1 to 4).
Evidently, therefore, the second term of the right member of the formula (1), .vertline.T.sub.g/2 -T.sub.SO .vertline., denotes the signal propagation delay time required to cover the distance between the personal station C and the personal station S and the third term, .vertline.T.sub.SO -T.sub.Rio .vertline., denotes the signal propagation delay time required to cover the distance between the personal station S and the personal station R.sub.i.
To be more specific, the personal station R.sub.3 which is located at a greater distance than the centrally located personal station C from the personal station S, for example, controls the aforementioned counters so that the second block #2, will be started on elapse of the time of the formula (2): EQU T.sub.b9 =T.sub.g/2 -T.sub.CR3 ( 2)
from the point of time at which the reception of reception S packets is completed. In this formula, T.sub.CR3 denotes the signal propagation delay time required to cover the distance between the personal station C and the personal station R.sub.3.
In the case of the personal station R.sub.2 which is located between the personal station S and the personal station C, the front guard time T.sub.b9, is calculated by the following formula (3). EQU T.sub.b9 =T.sub.g/2 +T.sub.CR2 ( 3)
In this formula, T.sub.CR2 denotes the signal propagation delay time required to cover the distance between the personal station C and the personal station R.sub.2.
In the case of the personal station R.sub.1 which is located at a greater distance than the personal station S from the centrally located personal station C, the front guard time T.sub.b9, is calculated by the following formula (4). EQU T.sub.b9 =T.sub.g/2 +T.sub.SC -T.sub.SR1 ( 4)
In this formula, T.sub.SC denotes the signal propagation delay time required to cover the distance between the personal station S and the personal station C and T.sub.SR1 that between the personal station S and the personal station R.sub.1.
As described above, in the digital signal transmission system the front guard time T.sub.b9, namely the interval between the time that the reception of packets sent out by the master station which takes the initiative in the frame synchronization (personal station S in the preceding illustration) (hereinafter referred to as "master packets") is completed and the time that the next block timing is started, is determined by each of the personal stations in accordance with the pertinent one of the aforementioned formulas, (1) through (4), using the relationship of position (or distance) with reference to the other personal station and the results are stored in the form of a conversion table in the memory device such as ROM (read only memory), for example.
This memory device is adapted so that when the code designating the transmitter of the master packet is injected into the memory device through the address designation terminal thereof, the memory device will feed out of its output terminal the signal corresponding to the front guard time T.sub.b9.
When this signal is fed into the programmable timer and the output of the timer is used to reset the aforementioned counters of a given personal station, this personal station is enabled to establish frame synchronization and block synchronization. The term "programmable timer" as used herein means a general-purpose circuit of the type which issues a signal after elapse of an interval equivalent to the aforementioned front guard time from the time at which the circuit is started by the injection of a signal corresponding to the front guard time T.sub.b9.
The adoption of such a method for the synchronization of system timing, however, evidently necessitates provision of conversion tables of dissimilar contents for the various personal stations. The contents of such conversion tables acceleratingly increase with the increasing number of personal stations involved in the system. The increase in the volume of contents of conversion tables, therefore, calls for great time and labor in the design and fabrication of memory devices such as ROM's which are used for storing such conversion tables. This fact has offered a great hindrance to the actual installation and operation of the communication system under discussion.
As a solution of this problem, there has been proposed a method which comprises causing distance codes which exclusively depend on signal propagation delay times between personal stations on the transmission cable to be included in advance in the address codes of packets to be sent out by personal stations and allowing the signal propagation delay time between two given personal stations to be calculated on the basis of the distance codes and the address codes.
In this method, the distance codes are transmitted as included in both the address number of the receiving personal station to which the packets are addressed and the address number of the transmitting personal station. At the receiving personal station, the calculation of the pertinent one of the aforementioned formulas (1) through (4) is performed on the basis of the two sets of distance codes.
In this case, since each of the personal stations is required to memorize distance codes of all the other personal stations against their corresponding address numbers, it must be provided with a memory device such as a ROM (read only memory) or RAM (random access memory), for example.
The provision of such memory devices for all the personal stations evidently entails a great addition to the cost of equipment. Moreover, when ROM's are adopted, a change in the location of any personal station necessitates replacement of the ROM's used at all the personal stations. This fact divests the system using ROM's of its economic feasibility.
When RAM's are adopted, a change made in the location of any personal station necessitates that particular personal station to cause the contents of the RAM's at all the other personal stations to be renewed wholly by the use of the transmission cable.
Further, the system must be provided with a proper backup power source (batteries) adapted to prevent the contents of such memory devices from being erased during a failure of the commercial power line.